Selective tuned circuits



April 20, 1943. R. A. WEAGANT 2,316,909

SELECTIVE TUNED CIRCUIT I Filed May 15, 1941 Usual flararzazwe IN'VENTOREoyJZWBay ATTORNEY Patented Apr. 20, 1943 2,316,909 SELECTIVE TUNEDcmcm'rs Roy A. Weagant, Douglaston, N. Y., assignor to Radio Corporationof America, a corporation of Delaware Application May 15, 1941, SerialNo. 393,532

10 Claims.

My present invention relates to high frequency tuned circuits of highdiscrimination between voltages of different frequencies.

One of the main objects of my present invention is to increase theeffect originally obtained with tuned circuits, and to producediscrimination effects which cannot be obtained through the employmentof solely coupled tuned circuits.

Another important object of my invention is to utilize a couplingnetwork, between cascaded high frequency circuits, arranged to transferthe high frequency energy of one circuit to the other in such a mannerthat a phase shift is produced when frequencies other than the resonantfrequency of the system are impressed upon the network.

Another object of the invention may be said to reside in the provisionof a tuned network which is constructed so that the current flow in aparticular part of the network is reduced due to frequencies oifresonance with the resonant frequency of the network, and the reductionbeing to a low value which is much less than that which corresponds tothe action of resonance alone.

Another object of the invention is to provide a network capable ofdistinguishing between the effects of voltages of difierent frequencies;currents being produced to correspond to the frequencies, and theeffects of the'currents being combined in a common circuit in a mannerwhich causes the amplitude of the current in the circuit to vary inaccordance with a phase angle between the aforesaid currents.

Still other objects of my invention are to improve generally theefficiency and selectivity of tuned signal circuits, and to providesignal selector networks which are reliable, durable and economicallymanufactured and assembled-in radio receivers.

The novel features which I believe to be char.- acteristic of myinvention are set forth in par.- ticularity in the appended claims; theinvention itself, however, as to both its organization and method ofoperation will best be understood,

by reference to the following description taken in connection with thedrawing in which I have indicated diagrammatically a circuitorganization whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows a circuit diagram of a receiver embodying the invention,

Fig. 2 illustrates the angular relations of the coupling coils of theinvention,

Fig. 3 shows a vector-ial explanation of the in: ent n,

Fig. 4 illustrates a resonance curve of a net: work employing theinvention,

Fig. 5 shows graphically the improvement due to the invention.

Referring now to the accompanying drawing, and particularly to Fig. 1,there is shown a superheterodyne receiving circuit which embodies theinvention. It is not believed necessary to describe the various circuitdetails of such a receiver, since those skilled in the art are fullyaware of such constructional details. The converter I may be of anydesired type, and, for example, may comprise a pentagrid tube whichfunctions as a combined oscillator and first detector circuit. Ofcourse, separate first detector and local oscillator tubes may beutilized. The signal collector may be of any desired type, and may becoupled to the tunable input circuit 2 of the converter through one ormore stages of radio frequency amplification. The tunable localoscillator tank circuit 3 may also be provided in the usual manner. Thedotted line 4 designates the customary tuning control mechanism employedbetween the rotors of the variable reactance elements of the signalcircuits and local oscillator tank circuit. Assuming that the receiveris used, by way of example, in the broadcast band of 550 to 170.0kilocycles (kc.), the local oscillator will be operated over a frequencyrange such that at each setting of the tuning control mechanism therewill be produced a constant intermediate frequency value such as, forexample, 465 kc.

In the output of the converter I there is arranged a resonant circuitcomprising coils II and I3 which are serially arranged between the plateof the first detector tube and the positive terminal of the directcurrent source which energizes the positive electrodes of the converter.The condenser I0 is shunted across coils II and I3. The network Hie-lle-l3 is tuned fixedly to the operating intermediate frequency (I. F.)value. As many I. 'F. amplifier tubes as is desired may be employed. Byway of simplicity only one such tube 20 is shown. This tube may be ofthe pentode type, although any other type may be utilized. The controlgrid 2| thereof is connected to ground for alternating currents througha path which comprises coil 3| and con-.- denser 3!. The condenser 32 isconnected in shunt with coil 3|, and fixedly resonates the latter to theoperating 1. F. value. The cathode of tube 20 may be connected to groundthrough l3 and M.

to the signal grids of the various I. F. amplifier tubes. This isexemplified by the connection of lead 56 to one end of coil 3| through afilter resistor 58. The lead 56 may be connected to any of the priorsignal grids in the usual manner. It will, therefore, be seen that thegrid return for grid 2| is through lead 56, resistor 55 and load.resistor 53 to ground.

The audio voltage developed across load resistor 53 is taken off by anadjustable potentiometer 6|) which is connected to ground through a se-The circuit 33-|4|6 is tuned to the operating I I. F. value, and thecircuit I5 is also tuned to the I. F. value. It will now be appreciatedthat signal voltages are transmitted to coil 3| through the pathincluding coil l3, and through a second path between coils II and I4.

Coils l3, 3| and H are preferably arranged in the manner of agoniometer. This type of coupling unit is well known in the art, and isshown in Fig. 2. Coils-| 3 and H! are arranged at right angles toprevent mutual coupling between them. The 'coil' 3| 'is arranged atapproxi mately 45 degrees between the planes of coils Hence, the-signalvoltage induced in coil 3| is the resultant of two signal voltagesinduced by the currents flowing in coils 3 and |4 respectively. It'will, therefore, beseen that whereas coils |3 and |4 are free of anymagnetic coupling, the coil -3| is coupled to each of coils :l3 and H.'Asis well known in goniometer construction, the coil'3l -may bearranged for adjustment relative'to the planes of coils 3 and 4. It willnow be seen that the tuned circuit 3| -32 is fed with I. F. signalsthrough tWo paths. One of these paths is the I. F.-tuned circuit||J-||-|3.. The second path is the cascaded series of I. F.-tun edcircuits which includes the tuned circuit l5 and the tuned circuit33--|6-|4. The amplifier has its plate connected to the high potentialside of an I. F.-tuned circuit whichcomprises coils and 4| arranged inseries in the'same manner as coils II and I3, the shunt condenser 42resonating the coils 40 and 4| to the operating'I. F. value. The linkcoupling circuit 43 is constructed in the same manner as couplingcircuit l5, and the'circuit 44 is constructed in the. same "manner astuned circuit 33--|6 +'|4. The coil 45 is magnetically coupled to thecoil 45, and coil 4| is also coupled to coil 45. Coils 4|, 46, and 45are arranged in the manner of a goniometer as in the case of coils|3-4||4. Coil 4 5 is shunted by a resonating condenser 41 which tunesthe coil to the operating I. F. value. In other words-the networkbetween the second detector and I. F. amplifier 20 is constructed in thesame manner as the network between the converter and the amplifier 20. VTube may be of the usual duplex diodetriod type. To preserve simplicityof description the duplex diode anode are shown as a single anodewhichis connected to one terminal of condenser 41. The cathode 5| isconnected to ground through a selfbiasing resistor 52 shunted by theusual I. F. by-pass condenser. The diode detection circuit is completedthrough the'load resistor 53 which is shunted by the I. F. by-passcondenser 54. Direct current voltage developed across load resistor 53is utilized for automatic volume controlpurposes. The AVC bias sodeveloped is applied through the filter resistor 55 to a lead 55, oneend of resistor 55 being connected to ground through the filtercondenser 51. The bias over lead 56 may be applied ries condenser andresistor. The control grid T0 of tube 50 is connected to any desiredpoint along the potentiometer resistor 6| by an adjustable tap. Thetriode section of tube 50 functions as an audio amplifier. triodesection may be connected to one or more audio frequency amplifiers whichmay be followed in turn by any desired type of audio reproducer. Sincethe network following the detector stage is very well known to thoseskilled in the art, it is not believed necessary to describe it in anyfurther detail.

For reasons which will be gone into in detail at a later point, thevoltages induced in each of coils 3| and 45 by their associated coils l3and I4 and 4| and 45 respectively, will have their phase relationsaltered as the input frequencies depart from resonance with the resultthat'their vector sum in each case will decrease with departure fromresonance. This means that the input to the detector stage will fall oifmore rapidly than would be the case if only the ordinary phenomena ofresonance was employed. At resonance, that is at the operating I. F.value, the voltages induced in coil 3| by each of coils l3 and M are tobe equal. This is also true in the case of coils 4|, 46 and 45.

Confining attention to the network between converter and I. F. amplifier20, the following explanation is given of the action which takes placein the system. It will be understood that the same explanation appliesto the coupling network between the amplifier 20 and the second detectorstage. When circuits |0| 3 and 3 |--32 are loosely coupled and inresonance with the intermediate frequency value, the current in circuit3|--32 will be in quadrature with the cur rent in circuit |0| ||3, orwill lag or lead as may be desired. Similarly, under the sameconditions, the current in circuit l5 will be in quadrature with thecurrent in the circuit ||l-| |-|3. A similar shift in phase will takeplace between circuit [5 and the circuit 33-|6|4. If these two shiftsare in the same direction, then the current in coil 3| will be shifteddegrees with respect to the current in coil 3. The direction ofconnection of coil I4 is so chosen that the voltage which it induces incoil 3| is in the same direction as that induced by the current in coill3. This means that at resonance with the I. F. value the two couplingeffects will add in so'far as coil 3| is concerned.

When the applied frequency departs from the I. F. value, this conditionno longer obtains. On the contrary, there occurs a shift in the relativephase relations of the voltages induced in coil 3| by the two circuitscoupled thereto, and an explanation of the action which then takes placeis as follows: Assume that the frequency departs from resonance ineither direction, then the current in circuit |0| |-|3 will shiftrelative to its phase at resonance. The voltage which is induced incircuit 3l-32 will shift by the same amount, while the current in thislatter circuit will also The plate of the.

shift relative to the induced voltage by the same amount.

From this it is evident that the current flowing in a circuit which isloosely coupled to a ,driving circuit changes in phase twice the angleof lead or lag of the current in the driving circuit. Continuing thisanalysis to circuit l5, we have at frequencies off resonance a currentflowing which lags or leads the voltage impressed on circuit Ill-I l--I3by an angle which is twice the phase angle between current and voltagein this latter circuit. Therefore, the voltage induced in circuit33+-.l6l,4 differs by this same angle from its phase at the resonantfrequency. The resultant current flow in circuit 33I 6-? will difierfrom the induced voltage by the original phase angle between current andvoltage in circuit lll-l l-..l 3, The voltage which this current willinduce in coil 3! is shifted from its resonant angle by this sameamount. This amount will be three times the phase angle between currentand voltage in circuit H1--Il |3.

Since the direction of phase shift in circuits Ill-l I-|3 and in circuit33-,I6-.-l4 is the same, while the amount of the latter is three timesthe former, the relative phase of the currents in these two circuits hasshifted by an amount which .is

- three minus one .or two times the phase angle between current andvoltage in circuit |-l |-l3.

This action takes places for frequencies on either side of the resonancefrequency. The inducing voltages supplied from coils l3 and I4 becomemore and more out of phase as the applied frequency departs from theresonance 1. F. value, and the resultant amplitude of the currentflowing in circuit 3 I-3 2 decreases.

In Fig. 3 there is shown a method for determining the amplitude of theresultant voltage induced in the circuit following the coupling network,and in this case that would be amplifier 20. In the diagram A representsthe phase angle between the voltages induced in coil 31, while ERrepresents the resultant voltage. The following expression may bewritten:

The vectors E and E1 represent the two induced voltages assumed to beequal in magnitude; A being the angle between them. Since the phaseangle between E and E1 is twice the phase angle in circuit l!l--l l-l3,the angle 13 which the resultant Ea makes with E is the aforementionedphase angle.

' Since the phase angle B may be expressed in terms of the circuitconstant as follows I the angle A may be expressed as 21rFL R equals the/3 and ER equals E or E1. If we assume an intermediate frequency of 465kc., and

all the circuits have an inductance of one thousand microhenries and aresistance of fifteen ohms, then it can be shown that the change .ofreactance per kc. of frequency is 12.18 ohms. To produce an angle B ofFig. 3 of 60 degrees a dee parture from resonance of 2.13 kc. is allthat is necessary, and it indicates that the amplitude of the voltagedelivered to the following circuit is cut in half at frequenciesapproximately 2 kc. each side of the resonance point. This statement isto be interpreted as m aning that the phase action herein describedreduces the value of the detector voltage to approximately half of whatit would be due to resonance alone.

It further indicates that in a radio receiver having an overallresonance curve of conventional form the selectivity is tremendouslyincreased, It indicates, further, that the required selectivity for thistype of reception is obtainable with an overall resonance curve which ismuch broader than that customarily employed. The consequenceof this isthat by means of the invention described herein the required selectivityfor any service may be obtained with a smaller number of tuned circuits,or with circuits which are inherently less efficient than is customary.

The phase selectivity action taking place between amplifier 20 and thesecond detector further increases the selective action. It will beunderstood that by the expression phase selectivity is meant theenhancement of the usual selective action of coupled resonant circuitsby virtue of the phase additive arrangement shown in this application.In Fig. 5 there are shown three curves which illustrate in a purelyqualitative manner the efiect of phase selectivity according to thepresent invention. The curve designed Usual resonance curve is the usualtype of single peaked resonance curve which would be obtained withloosely coupled cascaded tuned circuits, and would result, for example,if circuits l5 and 33-16-44 were omitted from the network between theconverter and amplifier 20. The second curve designated Result of phaseaction shows the type of selectivity curve that is secured by virtue ofthe introduction of circuits l5, and 33l 6,l4. This is the type of curvewhich is secured according to the method outlined in Figs. 3 and .4.

The innermost curve Resultant response curve is the product of the firsttwo curves. This shows the extent to which the usual resonancephenomenon has been enhanced by the present phase selectivity method. Ofcourse, the curves of Fig. 5 are purely qualitative, and are merely usedto show the nature of the increase in selectivity secured by means ofthe present invention. It is pointed out that one cf the importantadvantages of the present arrangement is that the persistence of thetuned circuits is not increased by this method. That is, even though theselectivity is greatly increased as shown in Fig. 5, the persistence ofthe cascaded tuned circuits is in no way increased. This is due to thefact that the voltages are induced in coil 3!, as well as in coil 46,iromsources which are additively related. That is, the present inventiondoes not affect the R, C or L constants ofthe tuned circuits. Anotherway of looking at this advantage is that the time constant of thenetwork is not increased, but remains that of a single tuned circuit.This is of particular advantage in reception of frequency, or phase,modulated carrier signals since circuits of minimum time constant valueare required in such cases.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand..described, but that many I network varies in accordance with therelative phase relations of the currents in the said two circuits andthe reactive couplings being so relatively phased that currents of saiddesired frequency are additively induced in said tuned network.

2. In combination. with a'plurality of cascadecoupled resonant circuitsall tuned to a common desired frequency, means for causing the currentflow in one of said'circuits to vary in accordance with a phasedisplacement between the currents of at least two other offsaid circuitsand said means comprising a separate reactive coupling between said onecircuit and each of the other two circuits, said couplings being relatedtoprovide additive induced'voltage in said one circuit at said desiredfrequency.

3. In a radio receiving system, a plurality of coupled resonantcircuits, means for tuning said circuits to a desired frequency, adetector following said circuits, and means for causing the amplitude ofthe currents impressed on the detector 'to decrease with departure fromresonance in accordance with the variations of a phase angle between twocomponents of the same frequency said means being additionally chosen tocause said impressed currents to be the additive effect of the currentsin said plurality of circuits at the said desired frequency.

4. In combination with a pair of electrical circuits which are tuned toa common frequency, means for shifting the relative phases of currentsproduced by voltages of different frequencies, and

means for combining these currents to produce 'an effect upon a currentor voltage indicator whose amplitude "decreases with departure from--said frequency said combining means being constructed to add currentsinduced from said pair of circuits at said common frequency.

'5. In combinati'on'witli a pair of resonant circuits arrangedincasc'ade, each of saidcircuits being tuned to a'coinmondesiredfrequency, a

coupling network providing a signal transfer path between said circuitswhich is auxiliary to a first 'couplingpath therebetween, said auxiliarypath :"being tuned tosaid common frequency, and signal vvoltagesbeing'induced in the last of the resoinant circuits in additive phase atsaid common frequency from the first circuit and from the auxiliarypath.

6. In combinationwith a tuned circuit reso- 'nant to a desired carrierfrequency of a modulated carrier Wave, a utilization circuit tuned tothe same carrier frequency, a first coupling path between said resonantcircuit and said utilization circuit for providing a desired selectivitycharacteristic, a second coupling path between said resonant circuit andsaid utilization circuit, said second coupling path being tuned to saidcarrier frequency and being so constructed and arranged that signalvoltages are induced in said utilization circuit from both said couplingpaths in'additive phase at said carrier frequency whereby theselectivity characteristic is made relatively sharper.

7. In combination with a tuned circuit resonant to a desired carrierfrequency of a modulated carrier wave, a second resonant circuit tunedto the same carrier frequency, a first-coupling path between said twotuned circuits for providing a desired resonance curve, a secondcoupling path between said two circuits, said second coupling path beingtuned to said carrier frequency and being so constructed and arrangedthatsignal voltages are induced in said second circuit from both saidcoupling paths in additive phase at said carrier frequency whereby thecurve is made relatively sharper.

8. In combination with a tuned circuit resonant to a desired carrierfrequency of a modulated carrier wave, a second resonant circuit tunedto the same carrier frequency, a firstcoupling path between said twotuned circuits for providing a desired resonance curve, 'a secondcoupling path between said two circuits, said second coupling path beingtuned to said carrier frequency and being so constructedand arrangedthat signal voltages are induced in said second circuit from both saidcoupling paths in additive phase at said carrier frequency wherebythe-curve is made relatively sharper and said second coupling pathcomprising at least two resonant circuits arranged in cascade betweensaid two tuned circuits.

9. In combination with a pair of resonant circuits arranged in cascade,each of said circuits being tuned to a common desired frequency, acoupling network'providing a signal transfer path between said circuitswhich isauxiliary to a first coupling path therebetween, said auxiliarypath consisting of at least two tuned circuits each tuned to said commonfrequency, and signal voltages being induced at said common frequency inthe last of the resonant circuits in additive phase from the firstcircuit and from the auxiliary path.

10. In combination with a first signal transmission tube, a tunedcircuit resonant to a desired carrier frequency o'fa modulated carrierwave, a utilization circuit tuned to the same car-.

rier frequency, a second tube having input electrodes coupled to theutilization circuit, a first coupling path between said resonant circuitand said utilization circuit for providing a desired degree ofselectivity, a second coupling path between said resonant circuit andsaid utilization circuit, said second coupling path being tuned to saidcarrier frequency and being so constructed and arranged that signalvoltages are induced in said utilization circuit from both said couplingpaths in additive phase at said carrier frequency whereby theselectivity is made relatively sharper.

ROY A. WEAGANT.

